Re: [PATCH 8/8] vmscan: Kick flusher threads to clean pages when reclaim is encountering dirty pages

[Wu Fengguang <fengguang.wu@intel.com>]

On Mon, Jul 19, 2010 at 09:11:30PM +0800, Mel Gorman wrote:
> There are a number of cases where pages get cleaned but two of concern
> to this patch are;
>   o When dirtying pages, processes may be throttled to clean pages if
>     dirty_ratio is not met.
>   o Pages belonging to inodes dirtied longer than
>     dirty_writeback_centisecs get cleaned.
> 
> The problem for reclaim is that dirty pages can reach the end of the LRU
> if pages are being dirtied slowly so that neither the throttling cleans
> them or a flusher thread waking periodically.
> 
> Background flush is already cleaning old or expired inodes first but the
> expire time is too far in the future at the time of page reclaim. To mitigate
> future problems, this patch wakes flusher threads to clean 1.5 times the
> number of dirty pages encountered by reclaimers. The reasoning is that pages
> were being dirtied at a roughly constant rate recently so if N dirty pages
> were encountered in this scan block, we are likely to see roughly N dirty
> pages again soon so try keep the flusher threads ahead of reclaim.
> 
> This is unfortunately very hand-wavy but there is not really a good way of
> quantifying how bad it is when reclaim encounters dirty pages other than
> "down with that sort of thing". Similarly, there is not an obvious way of
> figuring how what percentage of dirty pages are old in terms of LRU-age and
> should be cleaned. Ideally, the background flushers would only be cleaning
> pages belonging to the zone being scanned but it's not clear if this would
> be of benefit (less IO) or not (potentially less efficient IO if an inode
> is scattered across multiple zones).
> 
> Signed-off-by: Mel Gorman <mel@csn.ul.ie>
> ---
>  mm/vmscan.c |   18 +++++++++++-------
>  1 files changed, 11 insertions(+), 7 deletions(-)
> 
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index bc50937..5763719 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -806,6 +806,8 @@ restart_dirty:
>  		}
>  
>  		if (PageDirty(page))  {
> +			nr_dirty++;
> +
>  			/*
>  			 * If the caller cannot writeback pages, dirty pages
>  			 * are put on a separate list for cleaning by either
> @@ -814,7 +816,6 @@ restart_dirty:
>  			if (!reclaim_can_writeback(sc, page)) {
>  				list_add(&page->lru, &dirty_pages);
>  				unlock_page(page);
> -				nr_dirty++;
>  				goto keep_dirty;
>  			}
>  
> @@ -933,13 +934,16 @@ keep_dirty:
>  		VM_BUG_ON(PageLRU(page) || PageUnevictable(page));
>  	}
>  
> +	/*
> +	 * If reclaim is encountering dirty pages, it may be because
> +	 * dirty pages are reaching the end of the LRU even though
> +	 * the dirty_ratio may be satisified. In this case, wake
> +	 * flusher threads to pro-actively clean some pages
> +	 */
> +	wakeup_flusher_threads(laptop_mode ? 0 : nr_dirty + nr_dirty / 2);

Ah it's very possible that nr_dirty==0 here! Then you are hitting the
number of dirty pages down to 0 whether or not pageout() is called.

Another minor issue is, the passed (nr_dirty + nr_dirty / 2) is
normally a small number, much smaller than MAX_WRITEBACK_PAGES.
The flusher will sync at least MAX_WRITEBACK_PAGES pages, this is good
for efficiency. And it seems good to let the flusher write much more
than nr_dirty pages to safeguard a reasonable large
vmscan-head-to-first-dirty-LRU-page margin. So it would be enough to
update the comments.

Thanks,
Fengguang

>  	if (dirty_isolated < MAX_SWAP_CLEAN_WAIT && !list_empty(&dirty_pages)) {
> -		/*
> -		 * Wakeup a flusher thread to clean at least as many dirty
> -		 * pages as encountered by direct reclaim. Wait on congestion
> -		 * to throttle processes cleaning dirty pages
> -		 */
> -		wakeup_flusher_threads(nr_dirty);
> +		/* Throttle direct reclaimers cleaning pages */
>  		congestion_wait(BLK_RW_ASYNC, HZ/10);
>  
>  		/*
> -- 
> 1.7.1